Excitation of large amplitude compressional ULF waves in the magnetosphere by solar wind pressure spikes and IMF southward turning

We present a case study of large amplitude, compressional Pc5 waves observed by GOES 13 and 15 at geosynchronous orbit near noon. These waves were excited near the arrival time of two consecutive solar wind dynamic pressure spikes at the magnetopause, the second of which was associated with a large and rapid southward turning of the Interplanetary Magnetic Field (IMF). Magnetopause oscillations at a similar frequency were observed by the THEMIS spacecraft in the post-noon magnetosphere, however without clear signatures of compressional waves. The analysis of the ground response near the GOES foot-points, located close to the Churchill line of magnetometer stations, revealed an increase in wave power following each spike, with slight changes in polarizations. Following the second pressure spike's arrival at the magnetosphere, we found evidence of Field Line Resonances in the Northward component of the magnetic field, which suggests coupling with the compressional waves observed at GOES. We investigated potential Ultra Low Frequency drivers, and identified the most likely driving mechanism to be the drift-mirror instability; lack of appropriate measurements however prevented us from confirming this hypothesis.